1. Field of the Invention
The present invention relates to an FM detector for detecting an FM (frequency modulated) signal, to reproduce an original signal.
2. Description of Background Information
In the field of FM demodulation, various methods have been proposed for demodulating an FM signal, especially a digitalized FM signal. For instance, techniques in this field are discussed in abstracts for a national meeting of Japan Electronics and Communication Society, Communication section, held in 1980, in an article No. S5-8 entitled "A study of various methods for constructing an FM detector of DSP (digital signal processing) type". The article concludes that, among the methods described, the quadrature detection is most suited for the application of LSI (large scaled integrated circuit) technique in view of its relatively simple construction and high performance.
However, in quadrature detection circuits a second harmonic component is inevitably produced. Therefore, in prior art circuits, it was necessary to determine each parameter of FM modulation such as the angular frequency of the FM signal, and the deviation of the angular frequency, so that the second harmonic component does not overlap the frequency bandwidth of the baseband component. In cases where the angular frequency is relatively low with respect to the bandwidth of the baseband component in such cases as the FM modulation of the video tape recorders or video discs, bandwidths of the baseband component and the second harmonic component become very close, in frequency, with each other. Therefore, a filter having very sharp frequency cut-off characteristic, and usually many filter stages have been required for separating the baseband component in conventional design. In addition, if an off-set of the angular frequency toward the high frequency side occurs, it has resulted in generation of the so-called Aliasing distortion due to an overlap between the second harmonic component and the baseband component.